A thief was an individual who took property which did not belong to them.
The First Doctor, Barbara, Ian, and Susan met Robin Hood, Little John, and Will Scarlet. The three were members of a group of outlaws who stole from both the rich and poor as a source of income. (PROSE: The Thief of Sherwood)
While in Napoleonic Russia, the First Doctor, Steven, and Dodo encountered a shapeshifter they called the Shape Thief. The Shape Thief changed into the Doctor and tried to steal the TARDIS. (AUDIO: Mother Russia)
The Third Doctor incorporated an anti-thief device into Bessie. He used the device at least once to aid in the recovery of the Recovery 7 landing capsule which had been stolen by two thieves. The device lost power later but when activated the anti-theft device had both thieves locked to Bessie. (TV: The Ambassadors of Death)
When he tried to visit his friends Lord and Lady Tubbs, the Tenth Doctor discovered a thief had tried to steal an ancient horologe, dropped it, and transported those in the immediate vicinity into a different kind of time. (COMIC: 13 O'Clock)
Lady Christina de Souza was a notorious thief looking for excitement to get away from her life. She first met the Doctor after having just stolen the Cup of Athelstan from the International Gallery in London. (TV: Planet of the Dead)